The present invention relates to a process for the lamination of thermoplastic films and, in particular, to the lamination of thermoplastic films by a heat lamination process.
Thermoplastic films are used in a wide variety of end uses and, in particular, are used extensively in the packaging industry. However, for some end uses, individual thermoplastic films may have deficiencies in one or more properties, for example, in permeability, processability, and/or heat sealability of the film. These deficiencies may frequently be overcome by the application of coatings to the film or by the lamination of two or more films. Techniques for the manufacture of laminates are known, for example, the use of adhesives, co-extrusion, melt coating, and heat lamination under the influence of pressure. These techniques may require, for example, the use of expensive apparatus, the use of solvents, with the resultant fire, toxicity, and/or pollution hazards, and/or may be inefficient for the production of a range of laminates on one apparatus. There is, at the present time, increasing concern over the effects of solvents retained in packaging films, especially laminates manufactured by techniques requiring the use of solvents, and over the effects of solvents released to the atmosphere as the result of the use of such techniques. Techniques for the manufacture of laminates in the absence of solvents are therefor of increasing interest.
Relatively low-melting polymer films, for example, films of homopolymers of ethylene and copolymers of ethylene and butene-1, are frequently laminated to other thermoplastic films to enhance the heat sealability of these thermoplastic films. Laminates made with such low-melting films on an outer surface may be difficult to manufacture, in the absence of adhesives and/or solvents, on apparatus capable of being used economically for the manufacture of a wide range of laminates. In particular, it may be difficult to manufacture laminates having peel strengths of commercially acceptable levels between the films of the laminate on such apparatus.
As stated hereinabove, techniques for the heat lamination of films are known. A technique for the lamination of a polyolefin film to a substrate web, for example, paper, foil, regenerated cellulose film, and saran-coated regenerated cellulose film is described by A. L. James in U.S. Pat. No. 3,360,412, which issued on Dec. 26, 1967. The technique described by James is capable of improvement, especially for the lamination of two thermoplastic films, and, in particular, for the lamination of two thermoplastic films of similar softening or melting points and in the lamination of thermoplastic films in which a film of relatively low softening point comes in contact with a heated roll. The laminates formed by such a technique may not have peel strengths of commercially acceptable levels.